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Municipal sewage at a glance

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There are two entirely separate drainage systems for getting rid of unwanted water. Both rely upon gravity to flow with the natural gradient (however, topography or other features, may necessitate intermediate pumping stations):

Rain water from the roof, the yard and from the road is taken by large diameter pipes (to cope with cloud bursts) to the nearest natural water course.

Wastewater sewage from the toilet; waste wash water from the sink, bath, shower and the laundry is directed to the municipal sewage works via the municipal wastewater conveyance system, which is a network of sewers.

Sewage wastewater can be further split into:

Black water from the toilets, which is toxic and must never be touched, treated or consumed; andGrey water from the bath, shower and sink. This water can be cleaned for re-use in the garden.

Sewage management

Most local authorities split the management of wastewater into two separate divisions:

Conveyance

Gravity sewers, pump/lift stations and rising (pressure) mains combine to transport the sewage from your house or factory to the Wastewater Treatment Works (WwTW). Sewer diameter commences at 110mm and progressively increases as the anticipated volumes from confluencing sewer lines increase and may reach several metres in a large city.

Effluent stream before and after Bio-Systems treatment

Processing

The WwTW may be one of, or a combination of, bio-digestors, activated sludge plants and oxidation pond systems where sewage is screened to remove inorganic solids and then treated to degrade and remove organic impurities. Ultimately, the (previously dirty) water is returned sanitised – usually by chlorine contact to kill off any germs – to the natural environment - normally a natural water course (stream, river) but eventually the (inland) water table or the sea.